Golf players' foremost requirement for golf balls is high flight distance performance. In particular, golf players place importance on high flight distance performance upon a shot with a driver. High flight distance performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. With a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. With a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.
Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent approach performance. In particular, advanced golf players place importance on approach performance upon a shot with a short iron.
Golf players are further interested in feel at impact of golf balls. A hard cover deteriorates feel at impact. Golf players prefer soft feel at impact.
In light of achievement of various performance characteristics, golf balls each having a multilayer structure have been proposed. Japanese Patent Application Laid-Open No. 2007-319660 discloses a golf ball that includes a core, an envelope layer, a mid layer, and a cover. The mid layer of the golf ball is harder than the envelope layer and the cover thereof. Japanese Patent Application Laid-Open No. 2007-319667 and Japanese Patent Application Laid-Open No. 2008-68077 also disclose similar golf balls. Japanese Patent Application Laid-Open No. 2011-255172 discloses a golf ball that includes a central portion, a mid layer, and an outer layer. The outer layer of the golf ball is relatively hard and thick. U.S. Pat. No. 6,152,834 discloses a golf ball that includes a core and a cover having at least three layers. In the golf ball, a soft and thick cover is formed as an outermost layer.